The basic principle of the cable modem involves the modulation of digital signals to radio frequency (RF) signals and the demodulation of RF signals to digital signals that can be processed by a computer.
Referring to FIG. 1, the block diagram of a signal receiving assembly of a conventional cable modem, when downstream signal is inputted into the signal receiving assembly through a cable, the downstream signal is first received by a tuner and the frequency of the downstream signal is thereafter attenuated to an intermediate frequency (IF) by the tuner, and then the downstream signal is admitted into the modem chip for further processing, such as decoding. According to the power level magnitude of the downstream signal inputted into the chip, the chip generates two appropriate feedback signals AGC_T and AGC_I responding to the tuner and the IF amplifier respectively such that the tuner and the IF amplifier can adjust their automatic gain control values accordingly, thereby enabling the power level of the signal inputted into the chip can be controlled under a range that could be handled by the chip.
However, under the Data Over Cable Service Interface Specifications (DOCSIS) protocol, the frequency of downstream signal is required at a range of 88 MHz to 860 MHz, and under the circumstances, the tuner has an uneven frequency response. In addition, discontinuity points will be generated at the overlapping frequency bands, i.e. between low VHF and high VHF bands, or between high VHF and UHF bands. Furthermore, the frequency responses among different tuners are quite different such that each cable modem must be calibrated during production.
The conventional calibration method is first to select 26 frequency values and 18 power level values to generate 468 sampling signals by multiplying 26 by 18 and respectively reading the feedback signal AGC_T and AGC_I values of the modem chip under each sampling signal, thereby setting up a reference table containing 26×18 data for each modem. Thereafter, when the downstream signal is received by the modem, the power level can be calculated from the reference table, the generated values of the feedback signals AGC_T & ACG_I and the value of the frequency of the downstream signal. Please note that there are slight variances between actual inputted power level, but the error is in the tolerance of DOCSIS.
However, since the conventional calibration method requires that a detailed reference data table must be set up for each cable modem, the time spent on signal input and data reads for its set up is extremely lengthy, which results in an inordinate burden on the output rate of cable modem production lines.